Multiple access fiber optical bus systems are principally concentrated on the so-called "Tee" system as illustrated by U.S. Pat. No. 3,883,217 and the "Star" system as illustrated by U.S. Pat. No. 3,883,222.
The "Tee" system consists of a bus line accessed through "Tee" couplers. It essentially is only capable of down-the-line operation. In other words, terminals at the beginning of the bus can communicate only with those terminals which are down the bus route but the terminals downstream along the bus route can not communicate with the terminals at the beginning of the bus. While the "Tee" system provides easy access to such bus systems, it cannot accommodate a large number of terminals. This is due to the fact that each "Tee" has its coupling loss which is linearly additive with the number of couplers used.
On the other hand, the "Star" system consists of a "Star" point at which all terminal input and output lines terminate. At the "Star" point, a mixer is used to interconnect all terminals to each other. One such mixer is the so-called "Reflecting Star Coupler". That device accepts all input and output lines and transfers the signal from any input line to all the lines terminating there.
The loss of a reflecting "Star" coupler is given by the formula EQU 10 log.sub.10 2N + K
where N is the number of lines both input and output and K is a constant loss associated with the design of the device. For an ideal device K = 0 and in practice K may be as low as several dBs. Such a system has almost all the desirable characteristics in a bus system except for several important aspects and specifically access cannot be easily made to any point along the bus. Nor can terminals be replaced, removed or added with ease. Furthermore, it has the very serious disadvantage that if the reflecting "Star" coupler is destroyed that brings about a total system failure.
Thus, the "Star" system in configurations known to Applicant cannot be accessed at any point without prior planning in the design of the wiring harness and it is vulnerable to total system failure if the "Star" point is destroyed.
The desirable features of a fiber optical bus system are:
1. Multiple access to the bus.
2. The bus should be in the form of a loop so that each terminal has access to all other terminals.
3. Access can easily be made to any point along the bus.
4. Terminals may be replaced, removed or added with ease.
5. The system can be upgraded.
6. The system can accommodate a large number of terminals for example up to 100 or more and cover significant distances for example 1 km or more.
7. Failure of any one or more of the terminals should not effect the operation of the bus.
8. There should be no single vulnerable element which, is destroyed, causes system failure.
9. The fiber optical bus system should have a low cost.
10. The fiber optical bus system should have low weight.
11. Servicing of the optical bus system should be easy.